SROA: Don't drop atomic load/store alignments (PR45010)

SROA will drop the explicit alignment on allocas when the ABI guarantees
enough alignment. Because the alignment on new load/store instructions
are set based on the alloca's alignment, that means SROA would end up
dropping the alignment from atomic loads and stores, which is not
allowed (see bug). For those, make sure to always carry over the
alignment from the previous instruction.

Differential revision: https://reviews.llvm.org/D75266